JP2010230653A - Lightwave interference measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive and accurate optical interference measuring apparatus. <P>SOLUTION: This optical interference measuring apparatus includes a first multiple-wavelength light source 200a emitting a light beam having a plurality of spectra, a second multiple-wavelength light source 200b emitting a light beam having a wavelength different from that of the light beam from the first multiple-wavelength light source, a polarizing beam splitter 6 separating the light beams from the first multiple-wavelength light source 200a and second multiple-wavelength light source 200b, a reference surface 7 reflecting the light beam from the second multiple-wavelength light source 200b, a test surface 8 reflecting the light beam from the first multiple-wavelength light source 200a, spectral optical elements 9a and 9b dividing interference signals of the light beams from the first multiple-wavelength light source 200a and second multiple-wavelength light source 200b, detecting devices 10a and 10b which detect interference signals having single wavelengths of the light beams from the first multiple-wavelength light source 200a and second multiple-wavelength light source 200b for a plurality of frequencies, and an analyzer 11 for processing the signals from the detecting devices 10a and 10b and calculating the optical path difference between a reference plane 7 and a surface 8 to be inspected. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a light wave interference measuring apparatus.

  Conventionally, an optical interference measuring apparatus that measures an optical path length difference from a phase difference between a test signal and a reference signal has been proposed. For example, Patent Document 1 discloses a light wave interference measuring apparatus that measures the optical path length difference between a test optical path and a reference optical path using interference of two multi-wavelength light sources (optical frequency comb light sources) having different frequency intervals and center frequencies. ing. Patent Document 2 discloses an optical interference measuring apparatus that calculates a distance from a phase difference for each frequency component of a measurement signal and a reference signal.

  Non-Patent Document 1 discloses a method of dispersing an interference signal between a test optical path and a reference optical path generated from one multi-wavelength light source using a diffraction grating. In the method disclosed in this document, an interference signal corresponding to each spectrum of a multi-wavelength light source is separated, and a change in phase due to an optical path length difference with respect to frequency is homodyne-measured. In Patent Document 2, a frequency shifter is inserted in one of the test optical path and the reference optical path as a method of measuring the interference signal between the test optical path and the reference optical path by a diffraction grating with one multi-wavelength light source. Thus, a method of performing heterodyne detection is disclosed.

Japanese Patent No. 3739987 JP 2009-25245 A

Ki-Nam Joo & Seung-Woo Kim (2006). “Absolute distance measurement by dispersive interferometry using a femtosecond pulse laser” OPTICS EXPRESS, Vol. 14, No13, pp. 5954-5960.

  However, the conventional optical interference measuring apparatus using two multi-wavelength light sources having different frequency intervals and center frequencies has the following two problems. First, the measurement accuracy of the optical interference measuring apparatus is sensitive to the frequency stability of the two multi-wavelength light sources, in particular, the difference in the frequency interval between the multiple wavelength light sources, so it is difficult to improve the accuracy of the optical interference measuring apparatus. . Secondly, the conventional optical interference measuring apparatus requires two optical frequency comb generators, and further requires a complicated reference oscillator unit for stabilizing the frequency difference between the multi-wavelength light sources with high accuracy. Therefore, the light wave interference measuring apparatus becomes expensive.

  In particular, in the case of an optical interference measuring apparatus using one multi-wavelength light source, the phase detection accuracy of the interference signal is low because of homodyne detection, and high-precision distance measurement is difficult. In addition, even when heterodyne detection is performed by inserting a frequency shifter in either the test optical path or the reference optical path, the optical path length difference between the test optical path and the reference optical path is widened by the insertion of the frequency shifter. It is difficult to improve the accuracy of the apparatus. Furthermore, assuming a multi-axis application in which a plurality of interferometers are provided in one light source, a frequency shifter is required for each interferometer, so that the apparatus becomes expensive.

  Therefore, an object of the present invention is to provide an inexpensive and highly accurate optical interference measuring apparatus.

  An optical interference measuring apparatus according to one aspect of the present invention includes a first multi-wavelength light source that emits a light beam having a plurality of spectra, and a light beam that has a wavelength different from that of the light beam from the first multi-wavelength light source and orthogonal polarization components. A second multi-wavelength light source that emits; a polarizing optical element that separates a light beam from the first multi-wavelength light source and a light beam from the second multi-wavelength light source; and a reference position; A reference surface that reflects the light beam from the first multi-wavelength light source, a test surface that reflects the light beam from the first multi-wavelength light source, a light beam from the first multi-wavelength light source, and the second multi-wavelength light source. A spectral optical element that spectrally separates the interference signal of the luminous flux, and an interference signal that is spectrally separated by the spectral optical element, from the single wavelength of the luminous flux from the first multi-wavelength light source and the luminous flux from the second multi-wavelength light source. Detect interference signals for multiple wavelengths It has a number of detectors, an analysis apparatus and for calculating an optical path length difference between the reference surface by processing signals from the detector and the test surface.

  An optical interference measuring apparatus according to another aspect of the present invention emits a first multi-wavelength light source that emits a light beam having a plurality of spectra, and a light beam that has a polarization component orthogonal to the light beam from the first multi-wavelength light source. A second multi-wavelength light source; a wavelength control unit that periodically controls a frequency interval between a plurality of wavelengths of the first multi-wavelength light source; and a light flux from the first multi-wavelength light source for a test light flux and a reference light flux. A polarizing optical element that is divided into two; a reference surface that is provided at a reference position and reflects the reference light beam; a test surface that is provided on a test object and reflects the test light beam; the reference light beam; A plurality of spectral optical elements that split the interference signal of the test light beam and a plurality of wavelengths that detect interference signals between single wavelengths of the reference light beam and the test light beam from the interference signal that is spectrally separated by the spectroscopic optical element. Detectors from the detectors Wherein said reference surface by processing the No. and an analyzer for calculating the optical path length difference between the test surface.

  Other objects and features of the present invention are illustrated in the following examples.

  According to the present invention, an inexpensive and highly accurate optical interference measuring apparatus can be provided.

It is a block diagram of the optical interference measuring apparatus in 1st Embodiment. It is a flowchart of the measuring method performed by the analyzer in 1st Embodiment. It is a relationship figure of the phase difference and frequency of an interference signal in a 1st embodiment. It is a block diagram of the lightwave interference measuring device in 2nd Embodiment. It is a related figure of the optical path length and measurement wavelength in 2nd Embodiment. It is a block diagram of the optical interference measuring apparatus in 3rd Embodiment. It is a figure which shows the change of the etalon frequency interval in 3rd Embodiment. It is a block diagram of the lightwave interference measuring device by the homodyne detection in 3rd Embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each figure, the same members are denoted by the same reference numerals, and redundant description is omitted.
[First Embodiment]
First, the lightwave interference measuring apparatus according to the first embodiment of the present invention will be described. FIG. 1 is a configuration diagram of a lightwave interference measurement apparatus according to this embodiment.

  In the light wave interference measuring apparatus of the present embodiment, the first multi-wavelength light source 200a is configured to include the light source 1 and the optical frequency comb generator 2. The first multi-wavelength light source 200a emits a light beam having a plurality of narrow-band spectra (a plurality of narrow-band wavelengths). The second multi-wavelength light source 200b is configured by adding the frequency shifter 4 to the first multi-wavelength light source 200a, and emits a light beam having a plurality of narrow-band spectra different from the light beam from the first multi-wavelength light source 200a. To do. The light beam from the second multi-wavelength light source 200b has a polarization component orthogonal to the polarization component of the light beam from the first multi-wavelength light source 200a. The first multi-wavelength light source 200a and the second multi-wavelength light source 200b are optical frequency comb light sources in which each of a plurality of optical frequency components has an equal optical frequency difference. In addition, the first multi-wavelength light source 200a and the second multi-wavelength light source 200b modulate a light beam from a light source having a single narrow band spectrum by an optical modulation element, so that a plurality of light beams having a narrow band spectrum are obtained. Generate.

  The polarization beam splitter 6 is a polarization optical element that branches (separates) the light beam from the first multi-wavelength light source 200a and the light beam from the second multi-wavelength light source 200b. The reference surface 7 is provided at a predetermined reference position and is configured to reflect the light flux from the second multi-wavelength light source 200b. The test surface 8 is provided on the test object, and is configured to reflect the light beam from the first multi-wavelength light source 200a.

  The demultiplexers 9a and 9b are spectroscopic optical elements that split interference signals between the light flux from the first multi-wavelength light source 200a and the light flux from the second multi-wavelength light source 200b. The detection devices 10a and 10b have a plurality of detectors, and from the interference signals spectrally separated by the demultiplexers 9a and 9b, the light beams from the first multi-wavelength light source 200a and the light beams from the second multi-wavelength light source 200b, respectively. Interference signals between single wavelengths are detected for a plurality of wavelengths. The analysis device 11 processes signals from the detection devices 10 a and 10 b (a plurality of detectors) to calculate an optical path length difference between the reference surface 7 and the test surface 8.

  Hereinafter, the measurement principle in the optical interference measuring apparatus of this embodiment will be described in detail. A light beam having a single narrow bandwidth and having a center frequency stabilized with high accuracy and emitted from the light source 1 enters the optical frequency comb generator 2. The optical frequency comb generator 2 is configured by disposing an optical modulation element in a resonator, and performs phase modulation on incident light based on a modulation signal having a frequency fm from the oscillator 3. The light beam emitted from the optical frequency comb generator 2 becomes a comb-like spectrum with the frequency of the light source 1 as a center frequency and an interval fm, and becomes a multi-wavelength light source whose frequency interval is controlled with high accuracy by the oscillator 3. . The light beam emitted from the optical frequency comb generator 2 is partially transmitted by the half mirror and used as the first multi-wavelength light source 200a. Here, as a means for generating a multi-wavelength light source, a sideband may be generated using an electro-optic modulation element or an acousto-optic modulation element instead of the optical frequency comb generator 2. In this case, if a reduction in accuracy due to a decrease in the number of detection signals can be tolerated, it is advantageous in terms of cost.

  The light beam reflected by the half mirror enters the frequency shifter 4. In the frequency shifter 4, all of the spectrum generated by the optical frequency comb generator 2 is shifted by a uniform frequency df using an acousto-optic modulation element, and the polarization is changed so as to be orthogonal to the light beam emitted from the multi-wavelength light source 200 a. Rotate and inject. Since the frequency shifter 4 does not change the relative relationship of the spectrum, the luminous flux emitted from the frequency shifter 4 is also a multiwavelength in which the frequency interval of each spectrum is controlled with high accuracy by the oscillator 3 as in the first multiwavelength light source 200a. It becomes a light source. The light beam emitted from the frequency shifter 4 is used as the second multi-wavelength light source 200b.

  The light beams from the first multi-wavelength light source 200a and the second multi-wavelength light source 200b are branched into two by the non-polarizing beam splitter 5, respectively. Hereinafter, an optical path branched by the non-polarizing beam splitter 5 and incident on the branching filter 9a is referred to as a reference optical path, and an optical path branched by the non-polarizing beam splitter 5 and incident on the polarizing beam splitter 6 is referred to as a measurement optical path.

  In the measurement optical path, the light beam of the first multi-wavelength light source 200 a passes through the reflection surface of the polarization beam splitter 6. On the other hand, the light beam of the second multi-wavelength light source 200 b having a polarization component orthogonal to the first multi-wavelength light source 200 a is reflected by the reflecting surface of the polarization beam splitter 6. The light beam of the second multi-wavelength light source 200b reflected by the polarization beam splitter 6 is reflected by the reference surface 7 constituted by a corner cube composed of a plurality of reflection surfaces. The light beam reflected by the reference surface 7 is reflected again by the polarization beam splitter 6 and enters the branching filter 9b. Here, it is assumed that the reference surface 7 is fixed on a reference position that is a reference for distance measurement.

  On the other hand, the light beam of the first multi-wavelength light source 200a that has passed through the polarization beam splitter 6 is reflected on the test surface 8 fixed on the test object. The test surface 8 is formed of a corner cube like the reference surface 7. The light beam reflected by the test surface 8 passes through the polarization beam splitter 6 again and enters the branching filter 9b. Thus, in both the reference optical path and the measurement optical path, the light beams of the first multi-wavelength light source 200a and the second multi-wavelength light source 200b are split by the demultiplexers 9a and 9b.

  As the demultiplexers 9a and 9b, for example, arrayed waveguide diffraction type wavelength demultiplexers are used. In the following description, the arrayed waveguide diffraction type duplexer is referred to as AWG. The AWG is an element that demultiplexes by diffraction after emission of waveguides on an array having different optical path length differences, and is small and available at low cost. The duplexers 9a and 9b are required to have a wavelength resolution equal to or greater than the wavelength interval of the first multi-wavelength light source 200a and the second multi-wavelength light source 200b. The duplexers 9a and 9b are not limited to AWG, and for example, a bulk type diffraction grating may be used. When the resolution of the diffraction grating is insufficient with respect to the frequency interval of the optical frequency comb, the effective resolution can be increased by inserting an additional dispersion element such as an etalon in front of the diffraction grating. Depending on the specification wavelength and band, a bulk-type spectroscope may be configured at a lower cost than AWG. Further, a band-pass interference filter may be used as the duplexer of the present embodiment. When such an interference filter is used, there is an advantage that the configuration of the duplexer is simplified when the number of spectra of the multi-wavelength light source is small and the number of wavelengths to be measured is small.

  The outputs branched for each spectrum of the multi-wavelength light source from the demultiplexers 9a and 9b are received by detection devices 10a and 10b each having a plurality of detectors corresponding to the respective branches. The light received by the detection devices 10a and 10b is transmitted to the analysis device 11 as an interference signal between the first multi-wavelength light source 200a and the second multi-wavelength light source 200b. Here, in order to obtain an interference signal between the first multi-wavelength light source 200a and the second multi-wavelength light source 200b, a polarizer (not shown) as means for extracting a common polarization component of both light sources is incident on the demultiplexers 9a and 9b. Placed in front.

  In the present embodiment, only the polarization component in a predetermined direction is detected, but the interference signal is similarly detected by the duplexers 9a and 9b and the detection devices 10a and 10b for the component orthogonal to the detected polarization component. May be. At this time, differential detection is possible by inserting a wave plate in front of the polarizer so that the phases of the signals detected by both of the detection devices 10a and 10b are inverted. For this reason, measurement with higher accuracy is possible.

  Next, the content of the analysis performed by the analysis apparatus 11 of this embodiment is demonstrated. FIG. 2 is a flowchart of a measurement method executed by the analysis apparatus 11 in this embodiment.

First, in step S101, the analysis apparatus 11 measures the phase (interference phase) of the interference signal obtained for each spectrum (for each frequency) with respect to each of the reference optical path and the measurement optical path. That is, the analyzer 11 measures the interference phase between the reference surface 7 and the test surface 8 from each interference signal for a plurality of frequencies. The interference phase can be measured by configuring a phase meter. In order to detect the p-th wavelength branched by the demultiplexers 9a and 9b, the interference signal measured by the p-th detector in the detection devices 10a and 10b is measured by I ^ref _p as the interference signal of the reference optical path. If the interference signal of the optical path is I ^test _p , it is expressed as in equations (1) and (2).

Here, a1 _p and a2 _p are the amplitudes of the p-th frequency component of the first multi-wavelength light source 200a and the second multi-wavelength light source 200b, respectively. n ₁ L ₁ represents the optical path length difference between the optical path of the first multi-wavelength light source 200a and the optical path of the second multi-wavelength light source 200b before the non-polarizing beam splitter 5. n ₂ L ₂ is the optical path length difference of the optical path of the second multi-wavelength light source in the measurement optical path with respect to the reference optical path. Further, nD is the optical path length difference between the light beams from the first multi-wavelength light source 200a and the second multi-wavelength light source 200b after the polarizing beam splitter 6. Phase when the signal of frequency df detected by the phase meter, the phase of the signal in the reference optical path and the measurement optical path, respectively phi ^{ref _p,} when the phi ^test _p, is represented by the following formula (3) and (4) The

Next, the analyzer 11 calculates the phase difference between the signals on the measurement optical path and the reference optical path in step S102. The phase difference is obtained as shown in the following expression (5) by calculating the difference between the above expressions (3) and (4).

Next, the analysis apparatus 11 performs the first calculation of the optical path length difference nD between the reference surface 7 and the test surface 8 in step S103. Assuming that the refractive index dispersion is negligible, the optical path length difference nD is expressed by the following equation (6) using the change rate of the phase difference (interference phase) between the reference optical path and the measurement optical path for a plurality of frequencies. expressed.

FIG. 3 is a relationship diagram (actually measured value) between the phase difference of the interference signal and the frequency in the present embodiment. The optical path length difference nD is calculated by calculating the slope (rate of change) when the phase difference between the plurality of reference optical paths and the measurement optical path in FIG. 3 is linearly approximated with respect to the frequency. However, the measurement accuracy of the optical path length difference nD obtained by the above equation (6) is about several hundred nm even at a frequency fm of about 10 GHz, considering the measurement accuracy of about 10 ⁻⁵ rad of the phase meter. This accuracy is insufficient for use as the final output. Therefore, in the following, an analysis unit for calculating the optical path length difference nD with higher accuracy will be described.

First, the analysis apparatus 11 calculates the optical path length difference n ₂ L ₂ between the reference optical path and the measurement optical path in step S105. Since the optical path length difference n ₂ L ₂ is not a value that normally changes, it is not necessary to calculate each time. For this reason, as shown in FIG. 2, whether or not to calculate the optical path length difference n ₂ L ₂ is determined in step S104. From the above equation (5), when the optical path length difference n ₂ L ₂ is expressed using the optical path length difference nD ^meas1 of the equation (6), the following equation (7) is obtained.

Here, the overline in the equation (7) means an average value for p. As described above, since the calculation frequency of the optical path length difference n ₂ L ₂ is low, it is desirable to calculate the optical path length difference n ₂ L ₂ ^meas1 with high accuracy by averaging over a sufficient time. In addition, regarding the optical path length difference nD ^meas1 in Expression (7), it is possible to improve the accuracy by performing iterative calculation using the optical path length difference nD obtained in Expression (9) described later.

Next, in step S106, the analysis apparatus 11 calculates the interference order of the interference signal from the calculated optical path length difference. Here, the interference order means an integer that is multiplied by 2π in a component that is an integral multiple of 2π of the phase detected in the interference signal. paying attention to the phase difference between the p-th reference optical path and the measurement optical path in the spectrum of the interference order N _p of p-th interference signal is represented by the following equation (8).

Here, “round ()” represents a function for rounding an argument.

  Finally, the analysis apparatus 11 calculates again the optical path length difference nD between the reference surface 7 and the test surface 8 in step S107. The optical path length difference nD is calculated for each spectrum (for each of a plurality of frequencies) using the above-described interference order and the phase difference (interference phase) between the measurement optical path and the reference optical path. For this reason, the optical path length difference nD is expressed by the following equation (9) by averaging all the spectra for high accuracy.

Equation (9) is a calculation formula for the optical path length difference nD, but the geometric distance D may be calculated by dividing by the refractive index n as necessary. The refractive index n is obtained by measuring environmental conditions such as atmospheric pressure, temperature, and humidity in the vicinity of the optical interference measuring apparatus and calculating from the refractive index dispersion formula according to the measurement frequency. According to Expression (9), it is possible to calculate the geometric distance D with high accuracy at the ratio of f: fm with the same phase difference measurement accuracy as compared with Expression (6).

  In the present embodiment, the case where there is one light wave interferometer for the light source 1 has been described, but a plurality of light wave interferometers may be configured for one light source 1 when measuring a plurality of axes simultaneously. In this case, a necessary number of light beams are branched after the non-polarizing beam splitter 5, and an interferometer including the polarizing beam splitter 6, the reference surface 7, and the test surface 8 is arranged for each measurement axis. It is sufficient to add the demultiplexers 9a and 9b and the detection devices 10a and 10b.

As described above, according to the present embodiment, it is possible to provide an optical interference measuring apparatus that can perform distance measurement at low cost and with high accuracy.
[Second Embodiment]
Next, an optical interference measuring apparatus according to the second embodiment of the present invention will be described. FIG. 4 is a configuration diagram of the light wave interference measuring apparatus in the present embodiment.

  As shown in FIG. 4, the light wave interference measuring apparatus of the present embodiment is different from the first embodiment in the configuration of the light source unit. In the present embodiment, a broadband multi-wavelength light source 100a having a plurality of narrow-band spectra over a wide band is used as the first multi-wavelength light source. As the broadband multi-wavelength light source 100a, a femtosecond laser in which the carrier envelope offset frequency and the frequency interval are stabilized with high accuracy is used. In the case of a broadband light source, there is no frequency shifter that exhibits good conversion efficiency over the entire band. For this reason, another independent femtosecond laser is used as the broadband multi-wavelength light source 100b which is the second multi-wavelength light source.

  Here, the carrier envelope offset frequency of the broadband multi-wavelength light source 100b is controlled so as to stably maintain a constant frequency df difference with respect to the broadband multi-wavelength light source 100a. The oscillator 3 is a reference oscillator that controls the frequency interval of both the broadband multi-wavelength light sources 100a and 100b. As described above, the frequency interval between the broadband multi-wavelength light sources 100 a and 100 b is controlled by the same oscillator 3. Therefore, according to the present embodiment, it is possible to compensate for the oscillator-derived component of the frequency interval error between the light sources, which is one of the main causes of the distance measurement error. In the present embodiment, the configuration after the light source is the same as that in the first embodiment, and is omitted. Also for the analysis processing by the analysis apparatus 11, the optical path length difference between the reference surface 7 and the test surface 8 is calculated based on the flowchart shown in FIG.

In this embodiment, using the broadband characteristics of the broadband multi-wavelength light sources 100a and 100b, the geometrical difference in the optical path length difference between the reference surface 7 and the test surface 8 including correction of the refractive index variation of the measurement optical path is included. Analysis processing for calculating the distance is executed. The optical path length difference nD (f _p ) with respect to the frequency f _{p is} expressed by the following equation (10), where D is the geometric distance between the light beam reflected by the reference surface 7 and the light beam reflected by the test surface 8. It is represented by

In Expression (10), N _tp is a component that depends on the density of the medium in the non-identical optical path between the reference surface 7 and the test surface 8, and B (f _p ) is a function that depends only on the wavelength. Note that, as expressed by the equation (10), the known dispersion characteristic of the medium between the reference surface 7 and the test surface 8 is a component that depends on the density of the medium and a component that depends on the optical frequency component. It is approximated by the product and the sum of the refractive indices of the medium in vacuum.

  When the medium in the non-identical optical path between the reference surface 7 and the test surface 8 is air having a humidity of 0, the function B (f) is expressed by the following equation (11) using Edlen's equation.

In addition, what is necessary is just to set an appropriate function, when the medium between the reference surface 7 and the to-be-tested surface 8 differs from the above-mentioned conditions.

In order to correct the change in the medium density from the measurement result of the optical path length difference for a large number of frequencies, the analysis device 11 of the present embodiment fits a predetermined function to the measurement result of the optical path length difference to perform geometric analysis. The target distance D is calculated. Here, the predetermined function is represented by the sum of the change in the optical path length difference that varies depending on the geometric distance D and the refractive index of the medium between the reference surface 7 and the test surface 8. The change in the optical path length difference is obtained by multiplying the refractive index of a medium having a known dispersion characteristic between the reference surface 7 and the test surface 8 by the geometric distance D. When the measurement result of the optical path length difference is nD _meas (f _p ) and the predetermined function is D + N _tp · D · B (f _p ), the sum of squares of the fitting residuals is expressed as in equation (12).

The geometric distance D and N _tp · D that minimize Equation (12) are determined by solving a normal equation.

FIG. 5 is a relationship diagram between the optical path length and the measurement wavelength in this embodiment, and shows the result of fitting to the measured optical path length. Referring to FIG. 5, the geometrical distance D and the equation (12) represented by the alternate long and short dash line are optimized by approximation to the measurement result of the discrete optical path length represented by “◯”. N _tp · D can be obtained. When the refractive index of the medium between the reference surface 7 and the test surface 8 is necessary, the measurement result of the optical path length for each optical frequency is used as the geometric distance D between the reference surface 7 and the test surface 8. The refractive index can be calculated by dividing by.

As described above, according to the present embodiment, even when a multi-wavelength light source that requires independent frequency interval control is used, it is possible to perform synchronization with low cost and high accuracy by controlling the multi-wavelength light source based on the same oscillator. Become. Therefore, it is possible to provide a light wave interference measuring apparatus capable of performing high-precision distance measurement in which the influence of refractive index fluctuation is corrected without using a complicated reference oscillator unit.
[Third Embodiment]
Next, an optical interference measuring apparatus according to a third embodiment of the present invention will be described. FIG. 6 is a configuration diagram of a lightwave interference measuring apparatus in the present embodiment. The light wave interference measuring apparatus of this embodiment uses a multi-wavelength light source 300a including three semiconductor lasers 30a, 30b, and 30c as a measurement light source. Further, the semiconductor lasers 30a, 30b, 30c are stabilized by an etalon 31 as a single wavelength reference element.

  The semiconductor lasers 30a, 30b, and 30c are single longitudinal mode lasers having different oscillation wavelengths. By using a DFB laser as such a semiconductor laser, stable single mode oscillation can be realized at a relatively low cost. However, the present embodiment is not limited to this, and an external resonator type semiconductor laser (ECLD) or a surface emitting laser (VCSEL) may be used. Further, it is not necessary that the laser elements are independent from each other, and a type in which multiple wavelengths are integrated, such as a DFB laser for optical communication, may be used.

  Outputs from the semiconductor lasers 30a, 30b, and 30c are emitted by polarization-maintaining fibers, combined by a polarization-maintaining type multiplexer, and then split into two. One of the demultiplexed light beams is converted into a parallel light beam by a collimator and then enters an etalon 31 as a wavelength reference element. The etalon 31 is a vacuum gap type in order to avoid the influence of medium dispersion, and its temperature is controlled with high accuracy by the temperature controller 32. As a result, the vacuum wavelength and frequency interval of the transmission spectrum of the etalon 31 are compensated with high accuracy.

  In this embodiment, the etalon 31 is used as a vacuum gap. However, a solid gap etalon 31 can be used by compensating for dispersion. In this case, if an etalon of lithium niobate is employed, the optical path length can be modulated by the electro-optic effect. Therefore, it is possible to control the etalon frequency interval at a higher speed than temperature control, which is advantageous when modulating the etalon frequency interval described later. As a method of compensating the frequency interval of the etalon 31, as will be described later, a beat signal between two lasers stabilized in the adjacent transmission spectrum of the etalon 31 is measured, and the etalon is adjusted so that the beat frequency matches the reference oscillator. The frequency interval of 31 may be controlled. In this case, it is possible to compensate the frequency interval with high accuracy without depending on the long-term change of the etalon 31 due to stress release or the like.

  The light beam that has passed through the etalon 31 enters the branching filter 33, and the transmitted light amount corresponding to each of the semiconductor lasers 30 a, 30 b, and 30 c is detected by the wavelength controller 34. The wavelength controller 34 controls the injection current of the semiconductor lasers 30a, 30b, and 30c so that the transmittance of the light beams of the semiconductor lasers 30a, 30b, and 30c to the etalon 31 is constant. By such control, the wavelengths of the semiconductor lasers 30a, 30b, and 30c can be stabilized. In order to stabilize the wavelength with high accuracy, it is more preferable to measure not only the transmitted light amount of the etalon 31 but also the incident light amount. Alternatively, the laser wavelength may be stabilized with reference to the transmission spectrum peak of the etalon 31 by modulating the wavelength with a phase modulation element such as EOM and synchronously detecting the reflected light amount. In this case, since the transmission spectrum peak is used as a reference, highly accurate stabilization is possible without depending on the change in the transmission spectrum shape.

The etalon 31 stabilizes the frequencies of the semiconductor lasers 30a and 30b in adjacent transmission spectra, and stabilizes the frequencies of the semiconductor lasers 30a and 30c in transmission spectra separated from each other by N. That is, if the transmission spectrum interval of the etalon 31 is the first frequency interval FSR ₁ and the frequency of the semiconductor laser 30a is f ₀ , the frequencies of the semiconductor lasers 30b and 30c are f ₀ + FSR ₁ and f ₀ + N × FSR ₁ , respectively. It becomes. In the present embodiment, the multi-wavelength light source obtained as described above is the first multi-wavelength light source 300a.

  The light beam emitted from the first multi-wavelength light source 300a is branched into two, one light beam is transmitted through the frequency shifter 4, and the frequencies of the semiconductor lasers 30a, 30b, 30c are shifted by df. The light beam emitted from the frequency shifter 4 is rotated to polarized light orthogonal to the incident light beam by a wave plate (not shown). The light beam that has passed through the frequency shifter 4 is used as the second multi-wavelength light source 300b. The interference phase between the test surface 8 and the reference surface 7 is measured using the first multiwavelength light source 300a and the second multiwavelength light source 300b, as in the first embodiment.

Semiconductor lasers 30a, 30b, the phase phi _ref, the difference phi _test (phase difference) phi _a1 that is measured at each wavelength of 30c, φ _b1, when the phi _c1, between the reference surface 7 and the test surface 8 Is expressed by the following three formulas (13).

In Expressions 1 to 3 of Expression (13), the wavelengths are expressed as c / f ₀ , c / (N · FSR ₁ ), and c / FSR ₁ , and the phases are interference orders N, N ₁₃ , and N ₁₂ . It is expressed as the sum of the fractional phases φ _a1 , φ _c1 −φ _a1 , φ _b1 −φ _a1 . Here, the second formula and the third formula can be derived by applying the formula (6) to the two wavelengths of the semiconductor lasers 30a and 30c and the two wavelengths of the semiconductor lasers 30a and 30b, respectively.

If the measurement accuracy of the fractional phases φ _a1 , φ _c1 −φ _a1 , φ _b1 −φ _a1 is approximately the same, the measurement accuracy of the optical path length is better when the wavelength is shorter. For this reason, the first expression of Expression (13) has the highest accuracy, and the third expression has the lowest accuracy. On the other hand, the third equation is the longest and the first equation is the shortest in the unambiguous measurement range than the ambiguity of the interference order. However, there is a problem that the wavelength c / FSR ₁ of the third equation is limited to about 100 mm or less when the realistic first frequency interval FSR ₁ is set to several GHz.

Therefore, in the present embodiment, as shown in FIG. 7, the analysis apparatus 11 controls the temperature controller 32 to change the frequency interval of the etalon 31 by a minute amount. That is, the analysis device 11 and the temperature controller 32 (wavelength control unit) periodically change the frequency interval of the etalon 31 between the first frequency interval FSR ₁ and the second frequency interval FSR ₂ . The second frequency interval FSR ₂ is expressed by the following equation (14) using the minute interval dFSR.

When the difference (phase difference) between the phases φ _ref and φ _test measured at the respective wavelengths of the semiconductor lasers 30a, 30b, and 30c in the second frequency interval FSR ₂ is φ _a2 , φ _b2 , and φ _c2 , the optical path length difference nD is represented by the following two formulas (15).

When the minute interval dFSR is about 1 MHz, the longest wavelength c / dFSR reaches several hundreds of meters, and an unambiguous measurement distance can cover the entire measurement range in general applications. For this reason, the interference order is omitted in the second equation of equation (15). This means that absolute length measurement is realized. As described above, the longer the wavelength, the lower the measurement accuracy. Therefore, in the present embodiment, the five equations (13) and (15) are combined, and the interference order N in the first equation (13) is obtained sequentially. For this reason, it is possible to realize absolute length measurement while maintaining accuracy. A specific calculation formula is represented by Formula (16).

As shown in FIG. 7, the analysis device 11 changes the frequency interval FSR of the etalon 31 between the first frequency interval FSR ₁ and the second frequency interval FSR ₂ . Moreover, the analysis apparatus 11 performs absolute length measurement using Formula (16) from the phase measurement result in each frequency measurement result. That is, the analysis apparatus 11 calculates the optical path length difference again from the calculation result of the optical path length difference in the first frequency interval FSR ₁ and the calculation result of the optical path length difference in the second frequency interval FSR ₂ . When the modulation cycle of the frequency interval FSR is made high speed, absolute length measurement can always be performed even when the test light beam is frequently shielded.

  In this embodiment, the absolute distance measurement is always realized by periodically changing the frequency interval FSR of the etalon 31. However, in the first and second embodiments, the same effect can be obtained by changing the frequency of the oscillator. Obtainable.

  In this embodiment, heterodyne detection is performed using two multi-wavelength light sources (first multi-wavelength light source 300a and second multi-wavelength light source 300b), but homodyne detection is performed using a single multi-wavelength light source. May be. FIG. 8 is a configuration diagram of the light wave interference measuring apparatus in the case of homodyne detection. Compared with the optical interference measuring apparatus shown in FIG. 6, the optical interference measuring apparatus shown in FIG. 8A has a second multi-wavelength light source 300b, a branching filter 9a for measuring a reference signal, and a detecting apparatus 10a. The point that is deleted. Also, the phase measurement unit of the measurement signal is replaced for homodyne detection.

  As shown in FIG. 8A, the light beam from the first multi-wavelength light source 301a is separated by the polarization beam splitter 6 into two light beams, a test light beam and a reference light beam. The test light beam and the reference light beam are transmitted through a λ / 4 plate 40 having a fast axis angle of 45 degrees with each polarized light, and are converted into clockwise circularly polarized light and counterclockwise circularly polarized light. Thereafter, the light enters the spectroscopic optical element 41 and the light flux is divided for each wavelength of the semiconductor lasers 30a, 30b, and 30c. Then, the light beam is split into three light beams in the direction perpendicular to the paper surface by the grating beam splitter 42 and transmitted through the polarizer array 43, and then the light amount of the three light beams is detected independently for each wavelength by the detector array 44.

FIG. 8B is a detailed configuration diagram after the grating beam splitter 42 focusing on the wavelength of one semiconductor laser. The polarizer array 43 includes three polarizers whose transmission polarization axes are rotated by 120 degrees with respect to the light beam branched into three by the grating beam splitter 42. The three detectors 44a, 44b, 44c of the detector array detect the amount of light transmitted through each polarizer. When the phase of the measurement signal is φ, the light amounts I _a , I _b , and I _c detected by the detectors 44a, 44b, and 44c are expressed by Expression (17).

From the equation (17), the phase φ is calculated by the equation (18).

  The analysis apparatus 11 calculates the phase φ for the semiconductor lasers 30a, 30b, and 30c using the equation (18). The method of calculating the optical path length from the phase calculation result is the same as in the case of heterodyne detection. As described above, even when homodyne detection is used, the frequency interval of the etalon 31 is periodically changed, and the interference order is determined by using the equation (15) from the phase measurement result at each frequency interval. It is possible to realize length measurement.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

DESCRIPTION OF SYMBOLS 1 Light source 2 Optical frequency comb generator 3 Oscillator 4 Frequency shifter 7 Reference surface 8 Test surface 9a, 9b Divider 10a, 10b Detection apparatus 100a, 100b Broadband multi-wavelength light source 200a, 300a, 301a First multi-wavelength light source 200b, 300b Second multi-wavelength light source

Claims (11)

A first multi-wavelength light source that emits a light beam having a plurality of spectra;
A second multi-wavelength light source that emits a light beam having a wavelength different from that of the light beam from the first multi-wavelength light source and an orthogonal polarization component;
A polarizing optical element that separates a light beam from the first multi-wavelength light source and a light beam from the second multi-wavelength light source;
A reference surface provided at a reference position and reflecting a light beam from the second multi-wavelength light source;
A test surface provided on a test object and reflecting a light beam from the first multi-wavelength light source;
A spectroscopic optical element that splits an interference signal between a light beam from the first multi-wavelength light source and a light beam from the second multi-wavelength light source;
A plurality of detectors for detecting, for a plurality of wavelengths, interference signals having a single wavelength of the light flux from the first multi-wavelength light source and the light flux from the second multi-wavelength light source from the interference signal dispersed by the spectroscopic optical element. When,
An analysis device that processes a signal from the detector to calculate an optical path length difference between the reference surface and the test surface;
An optical interference measuring apparatus characterized by comprising:   2. The optical interference measuring apparatus according to claim 1, further comprising a reference oscillator that controls a frequency interval between both the first multi-wavelength light source and the second multi-wavelength light source.   The said spectroscopic optical element is an arrayed waveguide diffraction type | mold wavelength demultiplexer which has a wavelength resolution more than the wavelength space | interval of a said 1st multiwavelength light source and a said 2nd multiwavelength light source. The light wave interference measuring apparatus described. The analysis device includes:
Calculating an interference phase between the test surface and the reference surface from the interference signal for a plurality of frequencies;
4. The optical interference measuring apparatus according to claim 1, wherein the optical path length difference is calculated from a change rate of the interference phase with respect to the plurality of frequencies. 5. The analysis device includes:
Calculate the interference order of the interference signal from the calculated optical path length difference,
The optical wave interference measuring apparatus according to claim 4, wherein the optical path length difference is recalculated using the interference phase and the interference order for the plurality of frequencies. The analysis device calculates a geometric distance by fitting a function to the optical path length difference calculated again,
The function is represented by the sum of the geometric distance and a change in optical path length that varies depending on the refractive index of the medium between the reference surface and the test surface;
The change in the optical path length difference is obtained by multiplying a refractive index of a medium having a known dispersion characteristic between the reference surface and the test surface by the geometric distance. The light wave interference measuring apparatus according to any one of 1 to 5.   The first multi-wavelength light source and the second multi-wavelength light source are optical frequency comb light sources in which each of a plurality of optical frequency components has an equal optical frequency difference. The light wave interference measuring apparatus according to the item.   The first multi-wavelength light source and the second multi-wavelength light source generate a plurality of wavelengths by modulating a light beam from a single narrow-band wavelength light source with an optical modulation element. The light wave interference measuring apparatus according to any one of claims 6 to 6. Further comprising a single wavelength reference element;
The lightwave interference according to any one of claims 1 to 6, wherein each of the first multiwavelength light source and the second multiwavelength light source is stabilized by the single wavelength reference element. Measuring device. The analysis device includes:
The frequency interval between the first multi-wavelength light source and the second multi-wavelength light source is periodically changed between a first frequency interval and a second frequency interval,
The optical path length difference is calculated again from the calculation result of the optical path length difference at the first frequency interval and the calculation result of the optical path length difference at the second frequency interval. 2. The light wave interference measuring apparatus according to item 1. A first multi-wavelength light source that emits a light beam having a plurality of spectra;
A wavelength controller that periodically controls frequency intervals between a plurality of wavelengths of the first multi-wavelength light source;
A polarizing optical element for separating a light beam from the first multi-wavelength light source into a test light beam and a reference light beam;
A reference surface provided at a reference position and reflecting the reference light flux;
A test surface provided on the test object and reflecting the test light beam;
A spectroscopic optical element that splits an interference signal between the reference light beam and the test light beam;
A plurality of detectors for detecting interference signals of a single wavelength of the reference light beam and the test light beam for a plurality of wavelengths from the interference signal dispersed by the spectroscopic optical element;
An analysis device that processes a signal from the detector to calculate an optical path length difference between the reference surface and the test surface;
An optical interference measuring apparatus characterized by comprising: